1. Field of the Invention
The present invention relates to a spin valve magnetoresistance device and a method of designing the same.
2. Description of the Related Art
In recent years, higher sensitivity of magnetic sensors and the higher density in magnetic recording, have been sought. In particular, a magnetoresistance sensor ("MR sensor" hereinafter) and a magnetoresistance head ("MR head" hereinafter) using a mangetoresistance change are being actively developed. The MR sensor and the MR head is for reading an external signal on the basis of a change in the electric resistance of a magnetic field detection portion formed of a magnetic material. In reading data with a conventional inductive head, the output of the head decreases due to a decrease in the relative velocity of the head to a recording medium when the recording medium is downsized. However, the reproduction output of the MR head is not dependent upon its relative velocity to a recording medium, a high output is obtained even in high-density magnetic recording. The sensor portion of a conventional MR head is generally formed of Ni.sub.0.81 Fe.sub.0.19 (Permalloy), while the resistance change ratio of Permalloy is approximately 2% at the largest, and it is insufficient in sensitivity as a material for the MR head for reading ultra-high density magnetic record data of several GBPSI order.
In recent years, reports have been published on multi-layered films which exhibit a remarkably higher change ratio in magnetoresistance than a film of an alloy such as Permalloy. These are called an superlattices and have a structure in which thin layers having a thickness of an order of atomic diameter of a metal are periodically laminated. A Fe/Cr superlattice exhibiting a mangetoresistance change of over 40% at an ultra-low temperature (4.2K) has been reported (Phys. Rev. Lett. Vol. 61, page 2,472 (1988)). In the above superlattice, however, the external magnetic field (performance magnetic field intensity) where a maximum resistance change takes place is as large as tens of kilo oersteds, which is not practically usable. Further, a Co/Cu superlattice has also been proposed, but its performance magnetic field intensity is too large.
As a giant magnetoresistance change film which exhibits a large magnetoresistance change ratio, JP-A-4-358310 proposes a multi-layered film called a spin-valve type. A spin vale magnetoresistance device has a free layer which is a magnetic layer, a pinned layer which is a magnetic layer and a non-magnetic layer provided between them, and it is constituted so that antiparallel states of the two magnetic layers in magnetization are induced by an external magnetic field having an intensity between the coercive forces of the magnetic layers and the exchange interaction energy between the free layer and the pinned layer, whereby the device exhibits a large magnetoresistance change.
When many magnetic domains having various magnetization directions (regions having the same magnetization direction) are present in the free layer of the above spin valve magnetoresistance device, and when an external magnetic field is applied, the domains behave so as to align their magnetization directions with the directions of the external magnetic field. However, in this case, a problematic noise, called Barkhausen noise, is coupled with an output waveform.
For preventing the above Barkhausen noise, it is sufficient to keep the free layer so as to have a constant single magnetic domain. For this purpose, there has been already employed a method in which a permanent magnet is provided adjacently to the free layer. Specifically, there is known a configuration in which a spin valve magnetoresistance device having a track width of approximately 2.5 .mu.m is provided with a permanent magnet film having a thickness of approximately 30 nm and having a residual magnetization of approximately 800 emu/cm.sup.3.
It is also desired to further increase the recording density. For that purpose, it is known to narrow the track width. However, as the track width is narrowed, the reproduction output tends to decrease. According to the present inventors' calculation, for example, when the track width is decreased from 1 .mu.m to 0.3 .mu.m, the output becomes approximately 2/3. The reason for the above decrease in the output is assumed as follows. When an external magnetic field is applied to the free layer, the free layer does not entirely undergo magnetization turning, and both ends of the free layer have regions where no magnetization turns, so-called dead zones, due to the magnetic field of the permanent magnet used for controlling magnetic domains. The widths of the dead zones are dependent upon the intensity of the permanent magnet used for controlling magnetic domains but not dependent upon the track width. With a decrease in the track width, therefore, the ratio of the dead zone width to the total width of the entire free layer increases, and the output decreases.
For preventing a decrease in the reproduction output with decreasing the track width, one could decrease the intensity of the permanent magnet (residual magnetization) used for controlling magnetic domains within the limit in which the above Barkhausen noise is not caused.
However, the present inventors' studies have revealed that an inflection point appears in the reproduction output curve of the device or a hysteresis is caused in part of it even within the limit in which the Barkhausen noise is not caused, so that a desirable magnetoresistance change ratio can no longer be obtained.